The limb girdle muscular dystrophies (LGMDs) are genetically inherited diseases defined by progressive, proximal muscle wasting. Fifteen autosomal recessive LGMD disease genes have been identified, the majority of which link to the dystrophin glycoprotein complex and involve membrane fragility as a primary pathogenic feature. The most prevalent form of LGMD, LGMD2A, results from mutations in the gene encoding the muscle specific, calcium-dependent protease calpain 3 (capn3, C3). Unlike most other LGMDs, calpainopathy does not involve membrane damage as a primary feature; therefore, the mechanisms of this disease are quite different from other LGMDs and have remained mysterious since the gene defect was discovered in 1997. Over the last decade we have generated numerous genetically modified mice, which have been used to elucidate C3's biological role and to understand pathomechanisms of calpainopathy. This work has revealed a role for C3 in muscle growth and adaptation. Furthermore, muscles lacking C3 have reductions in the abundance and activation of the calcium calmodulin kinase (CaMK) signaling pathway at rest and upon muscle loading. The reductions in CaMK correspond to reduced slow fiber associated gene expression and a decreased number of slow fibers. In support of the relevance of these findings to calpainopathy, we also observed a primary involvement of slow fibers in LGMD2A biopsies. Thus, these studies are the first to identify defective CaMK signaling as the basis of the growth and adaptation defect in calpainopathy, and represent the first potential therapeutic target in LGMD2A. To define the relationship between C3 and CaMK, to identify biomarkers and to validate CaMK as a therapeutic target, we will carry out the following aims. Aim 1: We will test possible mechanisms by which calpain 3 modulates CaMK signaling. Aim 2: We will determine a transcriptional signature induced during muscle adaptation and determine the role of calpain 3 on that signature. Aim 3: We will create a bank of human myotubes derived from patients with different LGMD2A mutations, and then use this in vitro model to examine the effect of specific calpain 3 mutations on CaMK signaling and the gene signature identified in aim 2. Aim 4: We will determine if defects in CaMK signaling underlie the pathology of calpainopathy, and whether restoring downstream components of this signaling cascade improves the C3KO phenotype. Data generated from these efforts will provide fundamental insights into mechanisms and biomarkers and will reveal a novel therapeutic strategy for LGMD2A and may have far reaching implications for other diseases and conditions that impact muscle health, such as multi-minicore disease and sarcopenia that occurs with aging.